Body wave and surface wave analysis of large and great earthquakes along the Eastern Aleutian Arc, 1923–1993: Implications for future events

Abstract

The 500‐km‐long Alaska Peninsula and Shumagin Islands segments of the Aleutian arc have a moderate to high probability of rupturing in one or more large or great earthquakes in the next few decades. To understand the likely modes of rupture in the next sequence of large and great events and to delineate the current geometry of the plate interface, we determine focal mechanisms, depths, and source time functions from seismic records for the largest events since 1917: the great earthquake of November 10,1938, and seven events with surface wave magnitude (M_s) of 6.9 to 7.5. Teleseismic body waves and surface waves are used to estimate the seismic moment and gross rupture characteristics of the great earthquake of November 10,1938, along the Alaska Peninsula. Body wave inversion of five P and four SH waves gives a duration of about 110 s in which moment was released in two episodes, each of about 50 s duration, with the second being larger than the first. The first source was located in the general vicinity of the epicenter of the mainshock, and the second, which occurred about 60 s later, was centered about 180 km to the northeast. The body‐wave‐derived seismic moment is 3.7×10²¹ N m (M_w 8.3). We corroborate the body wave results by calculating surface waves by normal‐mode summation and comparing them with data at periods greater than 50 s. An adequate fit to observed seismograms is obtained for either a single point source or two point sources, with one located about 180 km northeast of the mainshock epicenter. Rupture in 1938 appears to have been confined to the Alaska Peninsula segment; uniform rupture into the Shumagin region is not supported by the data. The M_s 6.9 earthquake of May 13,1993, ruptured a small portion of the Shumagin gap. The earthquake of May 14,1948, (M_s 7.5) occurred on a shallow dipping thrust fault with a depth of about 31 km, not 60 km as originally suggested. Five M_s ∼ 7 events in the 1938 rupture zone have locations, depths, and mechanisms that define a shallow dipping (16°–19°) plate interface which shallows to 8° under Kodiak Island. Subduction of seamounts of the Gulf of Alaska seamount province may explain the location, moment, focal mechanism, and depth of the five M_s ∼ 7 events. The plate interface dips nearly uniformly between the Alaska Peninsula and Shumagin segments, indicating that segmentation, if any, is not controlled by the orientation of the plate interface. A slight warp of the plate interface may form the boundary between rupture zones of great earthquakes along the Kodiak and Alaska Peninsula segments of the arc and may explain why rupture zones of large to great earthquakes rarely cross this tectonic boundary.